FIG. 23 is a sectional view illustrating a distributed feedback (hereinafter referred to as DFB) semiconductor laser disclosed in, for example, Japanese Published Patent Application No. Sho. 62-45834. In the figure, reference numeral 201 designates an n type InP substrate. An n type InP lower cladding layer 202 is disposed on the substrate 201. An n type InGaAsP active layer 203 is disposed on the lower cladding layer 202. A p type InP first upper cladding layer 204a is disposed on the active layer 203. A diffraction grating comprising a periodic pattern of stripe-shaped p type InGaAsP layers 205a is disposed on the first upper cladding layer 204a. A p type InP second upper cladding layer 204b is disposed on the diffraction grating 205a and on the first upper cladding layer 204a. A p.sup.+ type InGaAsP contact layer 206 is disposed on the second upper cladding layer 204b. The double-heterojunction structure comprising the active layer 203, the lower cladding layer 202, and the upper cladding layers 204a and 204b is formed in a narrow stripe-shaped mesa. An n type InP mesa embedding layer 209 is disposed on the substrate 1, contacting opposite sides of the stripe-shaped mesa. A p type InP current blocking layer 210 and an n type InP current blocking layer 211 are successively disposed on the mesa embedding layer 209 at the opposite sides of the mesa. An insulating film 212 including a window opposite the stripe-shaped mesa is disposed on the top and side surfaces of the laser structure. A p side electrode 207 is disposed on the insulating film 212, contacting the InGaAsP contact layer 206 through the window in that insulating film 212. An n side electrode 208 is disposed on the rear surface of the substrate 201.
A description is given of the operation. In the prior art DFB laser, when a forward bias is applied across the p side electrode 207 and the n side electrode 208, holes and electrons are injected into the active layer 203 from the p side electrode 207 and the n side electrode 208, respectively, and these holes and electrons recombine to produce light. In this laser, the active layer 203 and the diffraction grating 205a which have relatively large refractive indices are interposed between the n type InP lower cladding layer 202, the p type InP first upper cladding layer 204a, and the p type InP second upper cladding layer 204b which have relatively small refractive indices, whereby a waveguide is produced. Therefore, the generated light travels through the active layer 203 and the diffraction grating 205a in the direction parallel to the active layer.
Further, since the stripe-shaped portions of the diffraction grating 205a are periodically present in the p type InP upper cladding layer, the effective refractive index is periodically changed in the array direction of the stripe-shaped portions of the diffraction grating 205a. If the array pitch of the diffraction grating 205a coincides with the period at which the generated light is subjected to Bragg reflection, only light having a wavelength that satisfies the condition of Bragg reflection is reflected in the waveguide, resulting in laser oscillation.
Process steps for fabricating the laser structure of FIG. 23 are illustrated in FIGS. 24(a)-24(d).
Initially, as shown in FIG. 24(a), there are successively grown on the n type InP substrate 201 an n type InP lower cladding layer 202, an n type InGaAsP active layer 203, a p type InP first upper cladding layer 204, and a p type InGaAsP layer 205, preferably by MOCVD.
A prescribed pattern is formed on the-p type InGaAsP layer 205 using the two-luminous-flux interference exposure method and, thereafter, the p type InGaAsP layer 205 is selectively etched by chemical etching or the like until the etching front reaches the first upper cladding layer 204a, whereby the p type InGaAsP layer 205 is divided into a plurality of stripe-shaped parallel ridges 205a, producing a diffraction grating (FIG. 24(b)).
Thereafter, a p type InP second upper cladding layer 204b is grown over the entire surface of the wafer by MOCVD to bury the diffraction grating 205a (FIG. 24(c)).
Thereafter, the laser structure is formed in a stripe-shaped mesa by etching. Then, an n type InP layer 209, a p type InP current blocking layer 210, and an n type InP current blocking layer 211 are successively grown on the substrate 201 at opposite sides of the mesa. Thereafter, a contact layer 206 is formed on the entire surface of the wafer.
To complete the laser structure of FIG. 23, a p side electrode 207 and an n side electrode 208 are produced on the contact layer 206 and the rear surface of the substrate 201, respectively.
In the prior art DFB laser shown in FIG. 23, however, when the n type InP cladding layer 204b is grown on the diffraction grating 205a, the diffraction grating gets out of shape due to mass transport, whereby the thickness and amplitude of the diffraction grating are reduced, resulting in difficulty in controlling the coupling constant that affects the intensity of the distributed feedback applied to light.
FIG. 25(a) is a perspective view, partially in section, illustrating an integrated semiconductor laser and light modulator disclosed in, for example, Journal of Lightwave Technology, Vol.8, No.9, 1990, pp. 1357.about.1362. FIG. 25(b) is a sectional view of a part of the structure of FIG. 25(a) along the resonator length direction of the semiconductor laser.
In these figures, reference numeral 301 designates an n type InP substrate with a (100) surface orientation. A light modulator 300a and a laser diode (hereinafter referred to as LD) 300b are integrated on the n type InP substrate 301. The LD 300b includes a diffraction grating 310, an n type InGaAsP light guide layer 302, an undoped InGaAsP active layer 303, an undoped InGaAsP buffer layer 304, and a p type InP layer 305. The light modulator 300a includes an undoped InGaAsP light absorption layer 306, an undoped InGaAsP buffer layer 307, and a p type InP cladding layer 308. An Fe-doped current blocking layer 311 is disposed on the p type InP cladding layer 308 and the undoped InGaAsP buffer layer 307 of the light modulator 300a and on the p type InP layer 305 of the LD 300b. An InGaAs contact layer 312 is disposed on the current blocking layer 311. Reference numeral 313 designates a p type dopant diffused region, and numeral 314 designates an SiN insulating film. Reference numerals 315 and 316 designate p side electrodes of the modulator and the LD, respectively, and numeral 317 designates an n side electrode common to the modulator and the LD.
Process steps for fabricating the optical device of FIGS. 25(a)-25(b) are illustrated in FIGS. 26(a)-26(i).
Initially, as illustrated in FIG. 26(a), a .lambda./4-shifted diffraction grating 310 with 240 nm pitch is formed on a prescribed region of the (100) surface of the n type InP substrate 301 where a laser diode is to be located (region B in the figure).
In the step of FIG. 26(b), an n type InGaAsP light guide layer 302 (.lambda. (wavelength)=1.3 .mu.m, 0.1 .mu.m thick), an undoped InGaAsP active layer 303 (.lambda.=1.57 .mu.m, 0.1 .mu.m thick), an undoped InGaAsP buffer layer 304 (.lambda.=1.3 .mu.m, 0.1 .mu.m thick), and a p type InP layer 305 about 1 .mu.m thick are successively grown on the (100) surface of the n type InP substrate 301 by liquid phase epitaxy (LPE). Thereafter, a photoresist film 320 is deposited on the p type InP layer 305, and a portion of the resist film 320 in a region where a light modulator is to be located (region A in the figure) is selectively removed using conventional photolithographic techniques.
In the step of FIG. 26(c), using the photoresist pattern as a mask, the p type InP layer 305, the undoped InGaAsP buffer layer 304, the undoped InGaAsP active layer 303, and the n type InGaAsP light guide layer 302 are selectively dry-etched to expose the surface of the substrate 301 in the modulator region A.
In the step of FIG. 26(d), an undoped InGaAsP light absorption layer 306 having a band gap energy corresponding to a wavelength (.lambda.) of 1.44 .mu.m and a thickness of 0.3.about.0.5 .mu.m, an undoped InGaAsP buffer layer 307 having a wavelength of 1.25 .mu.m and a thickness of 0.1.about.0.3 .mu.m, and a p type InP cladding layer 308 about 3 .mu.m thick are successively grown by hydride vapor phase epitaxy (VPE). Thereafter, a photoresist film 321 is deposited on the p type InP cladding layer 308 and patterned in a stripe shape extending in what becomes the light guide direction of the LD, using conventional photolithographic techniques.
In the step of FIG. 26(e), using the photoresist pattern as a mask, those epitaxial layers on the substrate 301 are selectively dry-etched to form a stripe-shaped mesa 325 having a width of 2 .mu.m. Thereafter, portions of the undoped InGaAsP light absorption layer 306, the undoped InGaAsP buffer layer 307, and the p type InP cladding layer 308 in the LD region are etched away. Also, a portion of the p type InP cladding layer 308 in the modulator region is etched away near the boundary between the modulator region and the LD region, forming a groove for electrical isolation 326.
In the step of FIG. 26(f), a high resistivity Fe-doped InP current blocking layer 311 is grown on the substrate 301 at opposite sides of the stripe-shaped mesa 325 and in the isolation groove 326 by VPE. Subsequently, an undoped InGaAs contact layer 312 is grown on the current blocking layer 311 by VPE.
In the step of FIG. 26(g), a dielectric film 330 is deposited over the contact layer 312, and stripe-shaped openings are formed in the dielectric film 330 opposite the modulator region and the LD region. Using this dielectric film as a mask, Zn is selectively diffused into the Fe-doped InP current blocking layer 311 and the undoped InGaAs contact layer 312 until the diffusion front reaches the stripe-shaped mesa 325, forming p type dopant diffused regions 313.
Thereafter, the InGaAs contact layer 312 is selectively etched leaving stripe-shaped portions opposite the modulator region and the laser region (FIG. 26(h)).
Then, an SiN film 314 is deposited on the stripe-shaped InGaAs contact layers 312 and on the Fe-doped InP layer 311, and openings 314a and 314b are formed in the SiN film 314 using conventional photolithography and etching techniques as shown in FIG. 26(i).
Finally, a p side electrode metal layer is deposited over the SiN film 314 contacting the contact layer 312 exposed in the openings 314a and 314b and, thereafter, the metal layer is patterned to form a p side electrode 315 for the light modulator and a p side electrode 316 for the laser diode. Further, a common n side electrode 317 is formed on the rear surface of the substrate 301, completing the optical device shown in FIG. 25 in which the semiconductor laser 300b and the light modulator 300a are monolithically integrated on the same substrate.
A description is given of the operation. In this optical device, since the band gap energy of the undoped InGaAsP light absorption layer 306 of the modulator 300a is larger than the band gap energy of the active layer 303 of the semiconductor laser 300b, light produced in the active layer 303 in the stripe-shaped mesa travels toward the undoped InGaAsP light absorption layer 306 of the light modulator 300a, and laser light is emitted from the cleaved facet of the light absorption layer 306. In this state, when no bias voltage is applied across the light modulator 300a, light traveling toward the front facet passes through the light absorption layer 306 and is emitted from the cleaved facet of the light absorption layer 306. Since the band gap energy of the light absorption layer 306 is larger than the band gap energy of the active layer 303 as described above, the laser light traveling through the modulator region is not absorbed by the light absorption layer 306. On the other hand, when a reverse bias is applied across the light modulator 300a with the n side electrode 317 on the plus side and the p side electrode 315 on the minus side, an electric field is applied to the light absorption layer 306, and the effective band gap energy of the light absorption layer 306 is reduced due to Franz-Keldysh effect as shown in FIG. 29, whereby the traveling laser light is absorbed by the light absorption layer 306, i.e., it is not emitted from the facet. In this way, the laser output is controlled by applying a reverse bias to the light modulator.
In the integrated semiconductor laser and light modulator shown in FIG. 25, the light absorption layer 306 of the modulator 300a and the active layer 303 of the LD 300b are different semiconductor layers having different refractive indices grown in different epitaxial growth steps. In addition, when the epitaxial layers 306, 307, and 308 of the light modulator 300a are grown, the thicknesses of these layers unfavorably increase in the vicinity of the boundary between the light modulator and the LD. Therefore, the absorption layer 306 of the light modulator is not smoothly connected to the active layer 303 and the light guide layer 302 of the LD, whereby reflection and scattering occur at the contact part, adversely affecting the efficiency of the optical coupling between the light modulator and the LD.
When selective growth is carried out using an insulating film as a mask, i.e., when a wafer is partially masked with an insulating film and crystal growth is carried out selectively on part of the wafer where the insulating film is absent, the thickness of the grown layer is increased in the vicinity of the boundary between the unmasked part and the masked part, i.e., so-called edge growth occurs. Such edge growth also occurs when crystal growth is carried out on a wafer having a step, i.e., difference in level, as shown in FIG. 26(d). That is, in FIG. 26(d), the thicknesses of the layers 306, 307, and 308 grown on the lower region of the wafer, i.e., the light modulator region A, increase in the vicinity of the step.
The optical coupling efficiency is significantly affected by the edge growth. The edge growth caused by the step of the wafer increases with an increase in the height of the step. In this prior art, structure the height of the step of the wafer is equal to the total of the thicknesses of the light guide layer 302, the active layer 303, the undoped InGaAsP buffer layer 304, and the p type InP layer 305, i.e., 1.3 .mu.m or more, so that considerable large edge growth occurs.
Not only the reduction in the optical coupling efficiency but also an uneven surface after the crystal growth, which adversely affects processing after the crystal growth, such as the formation of the ridge structure.
FIG. 27 is a sectional view schematically illustrating an integrated semiconductor laser and light modulator disclosed in "Institute of Electronics, Information and Communication Engineers, 1990 Spring National Convention Record, C-20, p.4-295". In FIG. 27, reference numeral 401 designates an n type InP substrate. A light modulator 400a and a laser diode 400b are integrated on the n type InP substrate 401. The substrate 401 includes a diffraction grating 410 in a region where the LD 400b is located. An n type InGaAsP light absorption and light guide layer 402 is disposed on the substrate 401 including the diffraction grating 410. An undoped InGaAsP active layer 403 is disposed on the n type InGaAsP layer 402 in the LD region. A p type InP layer 404 is disposed on the active layer 403. A p type InP cladding layer 405 is disposed on the n type InGaAsP layer 402 and on the p type InP layer 404. P type InGaAsP contact layers 406a and 406b are disposed on the cladding layer 405 in the modulator region and the LD region, respectively. A p side electrode 407 of the modulator is disposed on the contact layer 406a and a p side electrode 408 of the LD is disposed on the contact layer 406b. An n side electrode 409 common to the modulator and the LD is disposed on the rear surface of the substrate 401.
Process steps for fabricating this optical device are illustrated in FIGS. 28(a)-28(c).
Initially, a diffraction grating 410 is formed on a part of the substrate 401. Then, a light absorption and light guide layer 402 about 0.3 .mu.m thick, an active layer 403 about 0.15 .mu.m thick, and a p type InP layer 404 about 0.1 .mu.m thick are successively grown on the substrate by MOCVD (FIG. 28(a)). Thereafter, portions of the p type InP layer 404 and the active layer 403 in a region where the diffraction grating 410 is absent, i.e., the modulator region, are selectively etched away (FIG. 28(b)). Thereafter, a p type InP cladding layer 405 and a p type InGaAsP contact layer 406 are grown over the wafer (FIG. 28(c)).
A description is given of the operation. The operating principle of this optical device is identical to that of the optical device shown in FIG. 25. That is, when a forward bias is applied across the laser diode 400b with the p side electrode 408 on the plus side, carriers are injected into the active layer 403 and laser oscillation occurs. In this state, when no bias voltage is applied across the light modulator 400a, laser light traveling toward the front facet passes through the light guide and absorption layer 402 and is emitted from the facet of the layer 402. Since the band gap energy of the light guide and absorption layer 402 is larger than the band gap energy of the active layer 403, the laser light traveling through the light modulator region is not absorbed by the light guide and absorption layer 402. On the other hand, when a reverse bias is applied across the light modulator 400a with the n side electrode 409 on the plus side and the p side electrode 407 on the minus side, an electric field is applied to the light guide and absorption layer 402, and the effective band gap energy of the light absorption layer is reduced due to Franz-Keldysh effect as shown in FIG. 29, whereby the traveling laser light is absorbed by the light absorption layer, i.e., it is not emitted from the facet. In this way, the laser output is controlled by applying a reverse bias to the light modulator.
In the prior art optical device shown in FIG. 27, since the n type InGaAsP layer 402 serves both as a light absorption layer of the modulator and a light guide layer of the LD, the unwanted reduction in the efficiency of the optical coupling between the light modulator and the LD and the uneven surface of the wafer, which are seen in the optical device of FIG. 25, are avoided.
However, the optical device shown in FIG. 27 including the n type InGaAsP layer 402 serving both as a light absorption layer of the modulator and a light guide layer of the LD has the following drawbacks.
A light absorption layer of a light modulator must be depleted when the modulator is reversely biased. In addition, it must be a low carrier concentration layer (undoped layer) to avoid breakdown. Therefore, if the InGaAsP light absorption and light guide layer 402 satisfies the above-described conditions for the light absorption layer, a portion of the layer 402 serving as a light guide layer of the LD also has a low carrier concentration. The resistance of the LD increases by several ohms at the light guide layer, whereby the operating voltage of the LD is unfavorably increased.
Further, the band gap energy of the light absorption layer is about 0.05 eV higher than the band gap energy of the active layer of the LD. The reason is as follows. In order to modulate light, the light modulator must provide a band gap energy of the light absorption layer smaller than the band gap energy of the active layer by the band gap reduction effect achieved when a reverse bias is applied. Therefore, the difference in band gap energies between the light absorption layer and the active layer should not exceed 0.05 eV corresponding to the reduced amount of the band gap energy. As shown in FIG. 29, the absorption coefficient of the light absorption layer decreases as the wavelength of the light increases. However, since the decrease of the absorption coefficient is relatively gentle, even when light produced in the LD has a wavelength of 1.55 .mu.m (band gap energy: 0.8 eV) and no bias is applied to the modulator, the light is partly absorbed by the InGaAsP light absorption layer having a band gap energy of about 0.85 eV (wavelength: 1.46 .mu.m). Therefore, an absorption loss of some degree is inevitable in the light guide layer of the LD that also serves as the light absorption layer of the modulator. As a result, the threshold current of the LD is increased or the efficiency of the LD is reduced.
FIG. 30 is a perspective view of an integrated semiconductor laser and light modulator disclosed in Electronics Letters, 16th Jan. 1992, Vol.28, No. 2, pp.153-154. In the figure, reference numeral 501 designates an n type InP substrate. A light modulator 500a and a laser diode 500b are integrated on the substrate 501. The substrate 501 includes a diffraction grating 511 in a region where the LD 500b is located. There are successively disposed on the substrate 501, an n type InGaAsP guide layer 502, an n type InP spacer layer 503, an n type InP lower cladding layer 506, an intrinsic type (hereinafter referred to as i type) InGaAs/InGaAsP multi-quantum well (hereinafter referred to as MQW) layer 507, and a p type InP upper cladding layer 508. The lower cladding layer 506, the MQW layer 507, and the upper cladding layer 508 are formed in a stripe-shaped ridge. The top and opposite sides of the ridge are covered with a p type InP layer 509. A p.sup.+ type InGaAsP contact layer 510 is disposed on the p type InP layer 509 at the top of the ridge. An SiO.sub.2 insulating film 512 is disposed over the structure. A p side electrode 513a of the light modulator 500a and a p side electrode 513b of the LD 500b are disposed on the p.sup.+ type InGaAsP contact layer 510. An n side electrode 514 common to the light modulator and the LD is disposed on the rear surface of the substrate 501.
Process steps for fabricating the optical device of FIG. 30 are illustrated n FIGS. 31(a)-31(c).
Initially, a diffraction grating 511 is formed on a part of the substrate 501 where a DFB-LD-is to be located. Then, an n type InGaAsP guide layer 502 and an n type InP spacer layer 503 are grown over the entire surface of the substrate 501 including the diffraction grating 511. Thereafter, a pair of SiO.sub.2 films 520 with a 2 .mu.m wide gap between them are formed on the spacer layer 503 (FIG. 31(a)). The width of the SiO.sub.2 film 520 is about 10 .mu.m in the DFB-LD region and about 4 .mu.m in the modulator region.
In the step of FIG. 31(b), using the SiO.sub.2 films 520 as masks, an n type InP cladding layer 506, an i type InGaAs/InGaAsP MQW layer 507, and a p type InP cladding layer 508 are selectively grown on the spacer layer 503 by MOCVD. The respective grown layers 506.about.508 are thicker in the region sandwiched by the wider (about 10 .mu.m) portions of the SiO.sub.2 films 520 than in the region sandwiched by the narrower (about 4 .mu.m) portions of the SiO.sub.2 films 520. This result is attributed to the fact that species reaching the SiO.sub.2 masks 520 migrate to the unmasked region where the substrate is exposed and deposited on that region because no material deposition occurs on the SiO.sub.2 masks.
Thereafter, each of the SiO.sub.2 films 520 is etched by 1 .mu.m from the inside of the stripe along its length to increase the gap between the SiO.sub.2 films 520, and a p type InP layer 509 is selectively grown covering the MQW structure (FIG. 31(c)). Further, a p.sup.+ type InGaAsP contact layer 510 is selectively grown on the p type InP layer 509.
Thereafter, a portion of the contact layer 510 at the boundary of the LD region and the modulator region is etched away to provide high electrical isolation. Finally, p side electrodes 513a and 513b are formed in the modulator region and the LD region, respectively, and an n side electrode 514 is formed on the rear surface of the substrate 501 to complete the integrated DFB-LD and light modulator shown in FIG. 30.
A description is given of the operation. As described above, the MQW layer 507 in the DFB-LD region is thicker than the MQW layer 507 in the modulator region. In a quantum well layer, the effective band gap energy (E.sub.g) decreases as the thickness of the well layer increases. Accordingly, in the MQW layer 507, the band gap energy E.sub.g1 of the DFB-LD is smaller than the band gap energy E.sub.g2 of the modulator. When the DFB-LD is forward biased for continuous oscillation, since E.sub.g2 &gt;E.sub.g1, laser light (wavelength .lambda..sub.1 =1.24/E.sub.g1) is not absorbed by the modulator, i.e., it is emitted from the facet. On the other hand, when a reverse bias is applied across the light modulator, the exciton wavelength absorption edge shifts toward the long wavelength side due to quantum confinement Stark effect of the MQW layer, and the effective band gap energy E.sub.g'2 of the modulator is smaller than the band gap energy of the DFB-LD, i.e., E.sub.g'2 &lt;E.sub.g1, whereby laser light is absorbed by the light modulator and quenched. In this way, on and off switching of the laser light are controlled by varying the voltage applied to the light modulator.
In the prior art optical device shown in FIG. 30, however, the width of the upper portion of the stripe-shaped ridge fabricated between the SiO.sub.2 masks 520 is only 2.about.3 .mu.m, and patterning of the p side electrodes on such a narrow region is very difficult, resulting in poor reproducibility. Further, in the stripe-shaped ridge, as shown in the sectional view of FIG. 32 taken along the stripe direction of the ridge, the total of the thicknesses of the grown layers in the DFB-LD region is 1.5.about.2 times as thick as that in the modulator region, so that a step of 1.about.2 .mu.m height is formed at the boundary between the LD region and the modulator region. This step adversely affects subsequent processing, such as formation of electrodes. In addition, since the MQW layer serving as a waveguide layer has a step, transmission loss of guided light is unfavorably increased. This unwanted increase in the transmission loss due to the step of the waveguide MQW layer also occurs in the prior art optical device shown in FIG. 33(c).
FIGS. 33(a)-33(c) are diagrams illustrating the structure and production process of an integrated semiconductor laser and light modulator, disclosed in Electronics Letters, 7th Nov. 1991, Vol. 27, No. 23, pp.2138-2140. In FIG. 33(b), reference characters A and B denote enlarged views of semiconductor layers in the modulator region and the laser region, respectively.
In these figures, reference numeral 601 designates an n type InP substrate. A light modulator 600a and a laser diode 600b are integrated on the substrate 601. The substrate 601 includes a diffraction grating 607 in the LD region. An n type InGaAsP guide layer 602 is disposed on the substrate 601 including the diffraction grating 607. An InGaAs/InGaAsP multiple quantum well (MQW) layer 603 is disposed on the guide layer 602. A p type InP cladding layer 605 is disposed on the MQW layer 603. P type InGaAsP cap layers 606 are disposed on the cladding layer 605 in the modulator region and the LD region, respectively. P side electrodes 608 and 609 of the light modulator 600a and the LD 600b, respectively, are disposed on the respective cap layers 606. An n side electrode 610 common to the light modulator 600a and the LD 600b is disposed on the rear surface of the substrate 601.
A description is given of the production process.
Initially, as illustrated in FIG. 33(a), a diffraction grating 607 is formed on a prescribed region of the InP substrate 601 where an LD is to be located, and a pair of stripe-shaped SiO.sub.2 films 620 extending in what becomes the light guiding direction of the laser are formed on the InP substrate 601 at opposite sides of the diffraction grating 607. The size of each SiO.sub.2 film 620 is about 200 .mu.m.times.400 .mu.m, and the space between the SiO.sub.2 films 620, i.e.; the width of the region where the diffraction grating 607 is present, is about 200 .mu.m. A light modulator will be formed on a region of the InP substrate 601 where the diffraction grating 607 and the SiO.sub.2 films 620 are absent.
In the step of FIG. 33(b), an n type InGaAsP guide layer 602, an InGaAs/InGaAsP multiple quantum layer 603, and a p type InP cladding layer 605 are grown on the substrate 601 by MOCVD. During the MOCVD growth, since no semiconductor material is grown on the SiO.sub.2 films 620, a large quantity of species formed in the growth process reach the LD region between the SiO.sub.2 films. Therefore, the growing layers grow faster in the LD region than in the modulator region where the SiO.sub.2 films are absent. Thus, the grown layers 602, 603, and 605 in the LD region are 1.5.about.2 times as thick as those in the modulator region. That is, as illustrated in FIGS. 33(b)A and 33(b)B, the well layer 631b included in the MQW layer 603 in the LD region is thicker than the well layer 631a included in the MQW layer 603 in the modulator region and, therefore, the band gap energy of the MQW layer in-the modulator region is larger than that in the LD region.
Thereafter, a p type InGaAsP cap layer 606 is formed on the p type InP cladding layer 605, and a part of the cap layer 606 at the boundary between the LD region and the modulator region is etched away. Then, p side electrodes 608 and 609 are formed on the separated cap layers 606 in the modulator region and the LD region, respectively, and a common n side electrode 610 is formed on the rear surface of the substrate 601, completing the optical device shown in FIG. 33(c) in which a semiconductor laser 600a and a light modulator 600b are monolithically integrated on the same substrate.
A description is given of the operation. The InGaAs/InGaAsP MQW layer 603 serves as an active layer in the LD region and as a light absorption layer in the modulator region. When a forward bias is applied across the p side electrode 609 of the LD 600b and the common n side electrode 610, carriers are injected into the InGaAs/InGaAsP MQW layer 603, and laser oscillation occurs at a wavelength that is determined by the effective band gap energy of the MQW layer and the diffraction grating 607. The effective band gap energy of the MQW layer depends on the thickness of the well layer included in the MQW layer, that is, the effective band gap energy increases as the thickness of the well layer decreases. In the above-described selective growth using MOCVD, the well layer is thicker in the DFB-LD region than in the modulator region, so that the band gap energy E.sub.g1 of the MQW layer in the DFB-LD region is smaller than the band gap energy E.sub.g2 of the MQW layer in the modulator region. When no bias voltage is applied across the light modulator while the DFB-LD is forward biased to continuously oscillate, laser light (wavelength .lambda.1=1.24/E.sub.g1) is not absorbed in the modulator region because E.sub.g2 is larger than E.sub.g1. The laser light is emitted from the facet. On the other hand, when a reverse bias is applied across the light modulator, the exciton absorption edge is shifted toward the long wavelength side due to the quantum confinement Stark effect of the MQW layer, and the effective band gap energy E.sub.g'2 in the modulator region is smaller than the effective band gap energy E.sub.g1 in the DFB-LD region, whereby laser light is absorbed by the light modulator and quenched. Therefore, on and off switching of the laser light are controlled by varying the voltage applied to the light modulator.
FIG. 34 is a sectional view illustrating an integrated semiconductor laser and light modulator disclosed in Japanese Published Patent Application No. Hei. 4-100291. In the figure, reference numeral 701 designates an n type InP substrate. A light modulator 700a and a laser diode 700b are integrated on the InP substrate 701. An n type InP buffer layer 704 is disposed on the InP substrate 701. An n type InGaAsP guide layer 705 is disposed on the buffer layer 704. An InGaAs/InGaAsP MQW layer 706 is disposed on the guide layer 705. A p type InGaAsP guide layer 707 is disposed on the MQW layer 706. The p type InGaAsP guide layer 707 includes a diffraction grating 708 in the LD region. A p type InP cladding layer 710 is disposed on the p type InGaAsP guide layer 707 including the diffraction grating 708. Two p.sup.+ type InGaAsP cap layers 711 are respectively disposed on the cladding layer 710 in the modulator region and the LD region. P side electrodes 716a and 716b of the modulator 700a and the LD 700b, respectively, are disposed on the respective cap layers 711. An n side electrode 717 common to the light modulator and the LD is disposed on the rear surface of the substrate 701. Reference numeral 715 designates an SiO.sub.2 insulating film.
FIGS. 35(a)-35(j) are diagrams illustrating process steps for fabricating the optical device of FIG. 34, in which FIGS. 35(a), 35(b), and 35(e) are perspective views, FIGS. 35(c), 35(d), 35(f), and 35(g) are sectional views taken along the resonator length direction, and FIGS. 35(h), 35(i), and 35(j) are sectional views perpendicular to the resonator length direction.
Initially, as illustrated in FIG. 35(a), a pair of SiO.sub.2 films 720, each having a width of about 100 .mu.m, are formed on a prescribed region of the substrate 701 where an LD is to be located. A stripe-shaped region 721 sandwiched by the SiO.sub.2 films 720 is about 30 .mu.m wide. A light modulator will be located in a region of the substrate 701 where the SiO.sub.2 films 720 are absent.
In the step of FIG. 35(b), the substrate 701 is etched using the SiO.sub.2 films 720 as masks. The etching rate of the substrate in the stripe-shaped region 721 sandwiched by the SiO.sub.2 films 720 (LD region) is higher than the etching rate of the substrate in the other region (modulator region), so that the region 721 is etched deeper than the modulator region, resulting in a stripe-shaped groove 722. Thereafter, using the SiO.sub.2 films 720 as masks for selective growth, an n type InP buffer layer 704, an n type InGaAsP guide layer 705, an InGaAs/InGaAsP multiple quantum well layer 706, and a p type InGaAsP guide layer 707 are successively grown on the substrate 701 by MOCVD (first MOCVD process). During the first MOCVD process, since species produced in the growth process are not deposited on the SiO.sub.2 masks 720, a large quantity of the species reach the groove 722 between the SiO.sub.2 masks, so that the growth rate in the groove 722 is higher than the growth rate on the other region, i.e., the modulator region. As a result, those grown layers 704 to 707 are thicker in the LD region than in the modulator region. The resulting structure after the first MOCVD process is shown in a sectional view in FIG. 35(c).
After removal of the SiO.sub.2 films 720, a primary diffraction grating 708 having a pitch of 2400 .ANG. is formed on the guide layer 707 in the LD region (FIG. 35(d)). Thereafter, a pair of SiO.sub.2 films 723, each having a width of about 100 .mu.m, are formed on the guide layer 707 in the modulator region. A stripe-shaped region sandwiched by the SiO.sub.2 films 723, where the guide layer 707 is exposed, is about 30 .mu.m wide. Then, a p type InP cladding layer 710 and a p.sup.+ type InGaAsP cap layer 711 are successively grown over the wafer by MOCVD (second MOCVD process). These layers 710 and 711 are thicker in the region sandwiched by the SiO.sub.2 masks 723 (the modulator region) than in the region where the SiO.sub.2 masks 723 are absent (the LD region). The wafer, before the second MOCVD of process in which the cladding layer 710 and the cap layer 711 are grown, is thinner in the modulator region than in the LD region, and these layers 710 and 711 grown in the second MOCVD process are thicker in the modulator region than in the LD region, so that the thickness of the whole wafer after the second MOCVD process is uniform. The resulting structure is shown in a sectional view in FIG. 35(f).
As illustrated in FIG. 35(g), a part of the cap layer 711 at the boundary between the LD region and the modulator region is etched away. Thereafter, a stripe-shaped SiO.sub.2 film 724 extending in what becomes the resonator length direction of the laser is formed on the wafer. Using the SiO.sub.2 film 724 as a mask for selective etching, the respective semiconductor layers are etched in a mesa shape as shown in FIG. 35(h). Then, using the SiO.sub.2 film 724 as a mask for selective growth, a high resistivity Fe-doped InP layer 725 is grown on the InP substrate 701 contacting the opposite sides of the mesa (FIG. 35(i)). After removal of the SiO.sub.2 film 724, an SiO.sub.2 insulating film 715 is deposited over the wafer and patterned to form two contact holes in the modulator region and the LD region, respectively. To complete the structure of FIG. 34, p side electrodes 716a and 716b are formed in the modulator region and the LD region, respectively, and a common n side electrode 717 is formed on the rear surface of the substrate 701. FIG. 35(j) is a sectional view of the completed device taken along a plane perpendicular to the resonator length direction.
The operating principle of the optical device shown in FIG. 34 fabricated according to the above-described process steps is identical to those of the optical devices shown in FIGS. 30 and 33(c). That is, in the structure of FIG. 34, the InGaAs/InGaAsP MQW layer 706 serves as an active layer in the-LD region and as a light absorption layer in the light modulator region. When a forward bias is applied across the p side electrode 716b of the LD 700b and the n side electrode 717, carriers are injected into the InGaAs/InGaAsP MQW layer 706, and laser oscillation occurs at a wavelength determined by the effective band gap energy of the MQW layer and the diffraction grating 708. The effective band gap energy of the MQW layer depends on the thickness of the well layer included in the MQW layer 706, i.e., the band gap energy increases as the thickness of the well layer decreases. In the above-described selective growth using MOCVD, the well layer is thicker in the LD region than in the modulator region, so that the effective band gap energy E.sub.g1 of the MQW layer in the LD region is smaller than the effective band gap energy E.sub.g2 of the MQW layer in the modulator region. When no bias is applied across the light modulator and the DFB-LD is forward biased to continuously oscillate, the laser light (wavelength .lambda.1=1.24/E.sub.g1) is not absorbed in the modulator region because E.sub.g2 is larger than E.sub.g1. The laser light is emitted from the facet. On the other hand, when a reverse bias is applied across the light modulator, the exciton absorption edge is shifted toward the long wavelength side due to the quantum confinement Stark effect of the MQW layer, and the effective band gap energy E.sub.g'2 in the modulator region is smaller than the effective band gap energy E.sub.g1 in the LD region, whereby laser light is absorbed by the light modulator and quenched. Therefore, on and off switching of the laser light can be controlled by varying the voltage applied to the light modulator.
In the prior art optical devices shown in FIGS. 30, 33(a)-33(c), and 34, the MQW layer serves both as an active layer of the LD and a light absorption layer of the modulator, and the band gap energy of the MQW layer is varied by varying the thickness of the well layer included in the MQW layer. Therefore, an optimum design of the MQW structure for each of the active layer and the light absorption layer is impossible. For example, in a long wavelength quantum well LD, a quantum well active layer is desired to have about five well layers each having a thickness of 4-8 nm. When the total thickness of the quantum well layers is increased by increasing the number of the well layers or the thickness of each well layer, the light, confinement effect is encouraged too much, whereby an elliptic laser light beam, that is long in the direction perpendicular to the respective layers of the laser, is emitted. In this case, it is difficult to narrow the emitted laser light. On the other hand, in the light modulator, the quantum well light absorption layer is desired to have about 10 well layers each having a thickness of about 8 nm. If the well layer is too thick, the voltage required for shifting the absorption wavelength is increased. If the well layer is too thin, the shifting of the absorption wavelength is reduced. Further, if the number of the well layers included in the quantum well light absorption layer is too small, the light confinement-coefficient is reduced, and the extinction ratio is reduced.
As described above, the optimum design values for the quantum well active layer of the LD are different from those for the quantum well light absorption layer of the modulator. However, in the structure shown in FIG. 34, the well layer of the LD is inevitably 1.5.about.2 times as thick as the well layer of the light modulator. If the well layer of the LD takes the optimum thickness, the well layer of the modulator is much thinner than the optimum thickness. In this case, the shift of the absorption edge when the electric field is applied, which is in proportion to the biquadrate of the well layer thickness, is decreased or the light confinement coefficient in the well layer, which is in proportion to the well layer thickness, is reduced, resulting in insufficient light absorption that causes insufficient extinction characteristics.
On the contrary, if the well layer of the light modulator takes the optimum thickness, the well layer of the LD is too thick, so that improvement in LD characteristics due to the quantum well is not achieved. Further, since the thickness of the active layer is increased, the vertical mode of the light distribution does not take the fundamental mode.
Further, since the LD and the light modulator includes well layers of the same number, the degree of freedom in design is low, so that it is very difficult to solve the above-described problems. As a result, if characteristics of the LD are given priority, characteristics of the light modulator are sacrificed, and vice versa.
In the integrated semiconductor laser and light modulator shown in FIG. 34, the width of the SiO.sub.2 film 720 used as a mask for selective growth is as wide as 100 .mu.m, and the interval between the adjacent SiO.sub.2 films 720 is only 30 .mu.m, and polycrystalline material is unfavorably deposited on the SiO.sub.2 films during the selective growth, which makes it difficult to remove the SiO.sub.2 films after the selective growth. In addition, the thickness of the layer grown in the stripe-shaped groove 722 between the SiO.sub.2 masks 720 varies in the width direction of the groove 722. That is, the grown layer is thicker in the vicinity of the SiO.sub.2 masks 720 than in the center between the SiO.sub.2 masks 720.